Multiply Multi-Digit Whole Numbers
Help Questions
5th Grade Math › Multiply Multi-Digit Whole Numbers
A student multiplies $135\times 24$ and writes this work:
$$
\begin{array}{r}
\ \ 135\ \times\ 24\ \hline
\ \ 540\ \ \ 270\ \hline
\ \ 810\ \end{array}
$$
Multiplication relies on place value, and partial products must represent multiplying by ones and tens correctly. Which statement best describes the error?
The student should not add partial products; they should multiply the partial products together.
The student forgot the tens place value: $135\times 20$ should be 2,700, not 270.
The student misaligned the ones partial product; $135\times 4$ should start in the tens place.
The student added instead of multiplying; they should have found $135+24$ first.
Explanation
Multi-digit multiplication uses place value to correctly interpret and compute with positional digits. Partial products separate the multiplication into parts based on the multiplier's digits and their places. Aligning digits means shifting higher place partial products left to match their value, like adding zeros for tens. This connects to the area model by visualizing the product as summed rectangular areas from place value pairs. A misconception is confusing addition with multiplication in partial products, but they must be multiplied first. The algorithm works through the distributive property applied to place values. It generalizes because it systematically accounts for all interactions between digits' places.
Compute $63 \times 25$ using the standard algorithm. Which value is the correct product (showing that multiplying by 2 tens gives a partial product with a zero)?
The product is 315.
The product is 1575.
The product is 1625.
The product is 880.
Explanation
Multi-digit multiplication uses place value to handle digits in ones, tens, and beyond systematically. Partial products come from multiplying by each digit with its place value, like 63 × 5 and 63 × 20 for 63 × 25. Aligning involves placing the tens partial product shifted left, often with a trailing zero for clarity. This ties to the area model, dividing into rectangles such as 60 × 20, 60 × 5, 3 × 20, and 3 × 5, totaling 1575. A misconception is omitting the zero in the tens partial product, which might lead to misadding and wrong totals like 126 + 315 = 441 instead. The algorithm works because it builds the product layer by layer using place values. It ensures consistent results for various multi-digit scenarios.
A student tries to multiply $76 \times 52$ using the standard algorithm. Their work shows partial products of $76 \times 2 = 152$ and $76 \times 5 = 380$, then they add $152 + 380 = 532$. Which statement best describes the mistake using place value reasoning?
The student should have subtracted the partial products instead of adding them.
The student forgot that the 5 in 52 means 5 tens, so the second partial product should be $76 \times 50$, not $76 \times 5$.
The student added instead of multiplying, so they should have found $76 + 52$.
The student should not line up digits by place value when multiplying.
Explanation
Multi-digit multiplication uses place value to expand numbers like 52 into 5 tens and 2 ones, which helps identify errors in computation. Partial products should be 76 × 2 = 152 for ones and 76 × 50 for tens, but the student mistakenly used 76 × 5 instead. Aligning digits requires right-alignment by place value, with the tens partial product shifted to reflect the extra factor of 10. This connects to the area model, splitting the multiplication into areas for 76 × 50 and 76 × 2, summed together. A common misconception is ignoring the tens place multiplier, treating 5 as just 5 instead of 50, leading to a smaller product. The algorithm works by breaking down numbers via place values and multiplying distributively. It generalizes well because it consistently applies these principles to yield correct results across various multi-digit problems.
A class is making $28$ snack bags. Each bag needs $45$ pretzels. The multiplication is $45\times 28$. Which claim about using the standard algorithm is incorrect (think about place value and partial products)?
Multiplication relies on place value because the digits in 28 represent 20 and 8.
You can find $45\times 28$ by adding $45\times 8$ and $45\times 20$.
Because 2 is in the tens place, $45\times 2$ should be added without shifting since it is still just 2.
The partial product for the 2 in 28 should represent 2 tens, so it must be written one place to the left.
Explanation
Multi-digit multiplication uses place value to correctly value each digit's contribution, such as 2 in 28 representing 20. Partial products arise from multiplying by each component, like 8 ones and 2 tens, to build the total. Aligning digits involves shifting the tens product to align with its place value during addition. This relates to the area model, where place values create grid sections whose areas sum to the product. A misconception is adding without shifting for tens, treating it as a simple digit instead of a multiple of ten. The algorithm is reliable because it decomposes and reassembles based on place values. It generalizes to ensure correct products for any multi-digit combination.
A student multiplies $312\times 48$ using the standard algorithm. Because multiplication relies on place value, the 4 in 48 represents 4 tens, so the partial product for 4 must be written in the tens place. Which final product is correct?
1,497
360
12,480
14,976
Explanation
Multi-digit multiplication uses place value to handle digits in their correct positional weights during computation. Partial products are formed by multiplying by each digit of the multiplier, incorporating its place value. Aligning digits requires positioning each partial product according to the place value, shifting left for tens and higher. This ties into the area model, where the total area is the sum of sub-areas defined by place value breakdowns. A common misconception is treating all digits equally without place adjustments, resulting in misaligned sums. The algorithm works because it breaks down the problem into distributive property applications. It generalizes by scaling to more digits while preserving place value accuracy.
A student is multiplying $268 \times 35$ and writes these partial products: $268 \times 5 = 1{,}340$ and $268 \times 30 = 8{,}040$. Because multiplication relies on place value, the tens partial product must use 30, not 3. Which final product is correct when the partial products are combined?
8,045
9,390
9,380
1,610
Explanation
Multi-digit multiplication uses place value to expand 35 into 3 tens and 5 ones, ensuring proper scaling in partial products. Partial products are 268 × 5 = 1,340 and 268 × 30 = 8,040, combined to 9,380. Alignment in the algorithm is by place value from the right, with shifting for higher places. This connects to the area model, where total area sums 268 × 30 and 268 × 5 parts. One misconception is using 3 instead of 30 for tens, which ignores place value and reduces the product. The algorithm succeeds through distributive multiplication over place values. It generalizes effectively, providing a consistent method for accurate multi-digit products.
A teacher buys $24$ boxes of markers. Each box has $36$ markers. Use the standard algorithm to find the product $36 \times 24$. Which value is the correct product (showing that place value matters when multiplying by tens)?
The product is 864.
The product is 144.
The product is 1008.
The product is 720.
Explanation
Multi-digit multiplication uses place value to break down numbers into their ones, tens, and higher components for accurate computation. Partial products are created by multiplying the first number by each digit of the second number, accounting for the digit's place value, such as 36 × 4 and 36 × 20 for 36 × 24. In the standard algorithm, digits are aligned by writing the partial product for the tens place shifted one position to the left, often adding a zero at the end. This connects to the area model, where the rectangle is divided into sections like 30 × 20, 30 × 4, 6 × 20, and 6 × 4, summing to the total product of 864. A common misconception is forgetting to multiply by the tens value, leading to incorrect additions like just 36 × 2 + 36 × 4 = 144 instead of using 20. The algorithm works because it systematically combines these place-value-based products. This ensures reliable results for real-world problems, like finding total markers in 24 boxes of 36 each.
Compute $58\times 42$ using the standard algorithm. Make sure the partial product for the $4$ in $42$ represents $4$ tens (so it is lined up with the tens place). What is the product?
100
2,356
1,160
2,436
Explanation
Multi-digit multiplication uses place value to decompose numbers and multiply their components separately before combining. Partial products involve multiplying the entire top number by each digit of the bottom number, respecting their places like ones or tens. Aligning digits means placing the tens partial product starting from the tens column to reflect its value. This relates to the area model, where each partial product corresponds to an area of a rectangle segmented by place values. A misconception is aligning all partial products in the ones place, which disregards the tens multiplier. The algorithm works by ensuring each partial product is scaled by its place value factor. Overall, it provides a reliable way to compute products by honoring numerical structure.
A student is solving $39\times 47$ with the standard algorithm.
They correctly find $39\times 7=273$.
For the tens digit, they write $39\times 4=156$ and add it without shifting left.
Which option shows the correct product of $39\times 47$ when place value is used correctly?
1,113
1,560
1,833
429
Explanation
Multi-digit multiplication uses place value to interpret digits correctly, such as a 4 in the tens place meaning 40. Partial products come from multiplying by each digit separately, like by 7 ones and then by 4 tens, creating intermediate results. Aligning digits involves shifting the tens partial product left to account for the place value multiplier. This ties into the area model, where the total area is the sum of sub-areas representing products of place value pairs. One misconception is adding partial products without shifting, which treats tens as ones and underestimates the product. The algorithm works by correctly scaling and summing these components. It ensures precision across various number sizes by respecting place value rules.
A student is multiplying $203 \times 47$ and writes the partial products $203 \times 7 = 1{,}421$ and $203 \times 40 = 8{,}120$. Multiplication relies on place value because 40 means 4 tens. Which combined product is correct?
1,461
8,120
9,551
9,541
Explanation
Multi-digit multiplication uses place value to interpret 47 as 4 tens and 7 ones, allowing for separate multiplications that are later combined. Partial products are formed by multiplying 203 by 7 to get 1,421 and by 40 (4 × 10) to get 8,120, reflecting the tens place. Digits are aligned in the algorithm by place value, starting from the right, with the tens partial product shifted one space left. This ties into the area model, where the total area is the sum of rectangles for 203 × 40 and 203 × 7. A frequent misconception is adding partial products without considering the place value shift, which undervalues the tens contribution. The algorithm functions reliably because it decomposes the multiplier into place value terms and applies distribution. This approach ensures accuracy by fully accounting for each digit's positional weight in the final product.